An MRI apparatus utilizes a nuclear magnetic resonance phenomenon to measure various parameters of atomic nuclei (usually proton) in any desired regions of a subject. Typically, a high frequency magnetic field is applied to the subject placed in homogeneous static magnetic field to obtain a tomographic image of the region from nuclear magnetic resonance signals (echo signals) induced thereby. In order to select a specific region, a gradient magnetic field is applied together with the high frequency magnetic field and further, in order to provide correct positional information for the echo signals measured, it is necessary to correctly control the application time and intensity of the gradient magnetic field.
However, actually due to the gradient magnetic field application, currents with damping property are induced in a variety of structural bodies around gradient magnetic coils. Such current induced with damping property is called as eddy current, which shows a different time constant component depending on types of magnets used for static magnetic field generation. For example, a time constant of a permanent magnet type apparatus is about 1 sec. and a time constant of a superconductive magnet type apparatus reaches up to about 2 sec. These eddy currents generate magnetic field varying spatially and temporally and cause to deviate the gradient magnetic field sensed by nuclear spins in the subject from an ideal condition. Thereby, reduction of image quality such as image distortion, signal intensity reduction and ghosting is induced.
In order to prevent such image quality reduction due to eddy currents, a method has been developed, in which the spatially and temporally varying magnetic field is measured and the magnetic field variation is compensated. For example, JP-A-10-272120 discloses a technology in which a phantom is disposed in a static magnetic field space, after applying a test gradient magnetic field a high frequency pulse is applied, NMR signals caused thereby are measured, magnetic field variation due to eddy currents caused by the application of the test gradient field is measured along with discrete time lapse, compensation currents for compensating the magnetic field variation are respectively determined and an influence affected to the magnetic field by the gradient field applied at the time when taking an image of a subject is avoided.
In the above method, after applying the test gradient magnetic field, nuclear spins in the phantom are excited by high frequency (RF) pulses and phase encoded free induction decay signals (FID signals) are sampled in a predetermined period. The measurement of the FID signals is repeated by varying the phase encode amount and a set of data of two dimensions or three dimensions is obtained. The FID signals contain, other than the eddy currents induced by the test gradient magnetic field, influences of inhomogeneity of the static magnetic field and eddy current due to the phase encoding gradient magnetic field. Thus, by changing the polarity of the test gradient magnetic field, a similar measurement is performed to obtain another set of data, through taking difference of the two sets of data obtained in the two measurements, the influences of inhomogeneity of the static magnetic field and eddy current due to the phase encoding gradient magnetic field are avoided. Thus, a phase difference image, which only contains the influence of eddy currents induced by the test gradient magnetic field, is obtained. Based on the phase difference image, magnitudes of the eddy currents are resolved spatially and temporally and their time constants are calculated and compensation currents to be flown in the gradient magnetic field coils and shim coils are calculated.
As has been explained above, in the conventional method, the following operation is repeated, in that after applying the test gradient magnetic field, the RF pulses of non-selectivity are applied, the phase encoding is performed in three axial directions, the FID signals are sampled along time lapse and four dimensional data containing time axis are obtained.
However, since the FID signals measured with the conventional art decay according to lapse of time, a level of NMR signals for measuring an eddy current having a long time constant reduces according to time, namely, since S/N reduces according to lapse of time, it is afraid that the measurement accuracy of eddy currents having a long time constant will be reduced.
Further, the eddy currents induced by the application of the gradient magnetic field are caused when the gradient magnetic field rises and falls and differences in the induced currents at the time when rising and falling of the gradient magnetic field application is sometimes caused due to such as vibration of structural bodies of magnet disposed around the gradient magnetic field coils. However, the measurement object in the above conventional art is limited only to the eddy current components induced at the time when the gradient magnetic field falls, and the conventional art does not disclose to measure the influence to the magnetic field due to the eddy current component induced when the gradient magnetic field rises and to compensate the same.
An object of the present invention is to provide an MRI apparatus which measures the eddy current components having a long time constant induced when the gradient magnetic field rises and/or falls and permits to compensate with a high accuracy the influence to the magnetic field caused thereby.